This invention relates to an internal combustion engine control system and, in particular, to a charge forming device to effect split engine operation of a multi-cylinder internal combustion engine having a closed loop or feed back control system including an exhaust sensor.
A multi-cylinder internal combustion engine is known which has a three-way catalytic converter in an exhaust system and a feed back control system in which the quantity of fuel fed to the engine is controlled in response to the output from an oxygen sensor, which detects the oxygen concentration of the exhaust gases, in order to maintain the air fuel ratio of the mixture at the stoichiometry whereupon the three-way catalytic converter can operate most efficiently.
It is also known to operate a multi-cylinder internal combustion engine by suspending the supply of fuel to selected cylinders, reduced in number, of all to deactive the same and by increasing the load on the remaining activating cylinders to operate the same under high load conditions in which fuel economy is good.
This is known as means for improving fuel economy of the engine. Applying this means to a multi-cylinder internal combustion engine provided with a three-way catalytic converter and a feed back control system will cause the air fuel ratio of the exhaust gases to deviate considerably from the stoichiometry toward the lean side when the engine operating condition shifts to split engine operation mode because at this engine operation mode the exhaust gases discharged from the activating cylinders will be diluted with air discharged from the deactivated cylinders. As a result, the quantity of fuel supplied to the activating cylinders will be increased excessively in accordance with the output from the oxygen sensor which represents the oxygen concentration of the exhaust gases containing oxygen of air discharged from the deactivated cylinders, thereby to deteriorate the driveability and thus worsening the fuel economy.
Under split engine operating condition, the engine operates efficiently because the activating cylinders operate under high engine load. Therefore, the feed back control may be suspended under split engine operating condition because under this condition the exhaust emissions discharged from the engine cylinders are not at high levels.
If, in such a control as above, the feed back control is resumed concurrently when the engine operating condition shifts back into full cylinder operating mode from split operation mode, the quantity of fuel injection will excessively increase upon this shifting because the oxygen sensor detects the oxygen concentration of the exhaust gases resulting from the split operating condition of the engine for a predetermined time duration after the engine operating condition has shift back into full cylinder operation mode from split operation mode (ref. FIG. 1).